1. Field of the Invention
Applicant's invention relates to selective access systems for control of the use of motor vehicles.
2. Background Information
a. The Problem Presented.
Systems are known which are intended to control access to and/or operation of motor vehicles, such access or denial thereof intended to be determined by a would-be driver's alcohol level. Such systems involve the interfacing of a "breathalizer" with a vehicle's ignition system. Such an "interlock" system interrupts operation of the vehicle engine's ignition and/or starter systems if the breathalizer indicates that the user exhibits a blood alcohol level in excess of a pre-determined level. Conversely, if a lower than prescribed alcohol level is indicated, the vehicle will operate normally.
Examples of vehicle interlock systems for controlling vehicle operation by intoxicated drivers are taught in the following U.S. patents:
______________________________________ U.S. Pat. No. Inventor(s) ______________________________________ 3,780,311 Brown 4,738,333 Collier 4,607,719 Rugis, et al 4,613,845 DuBois ______________________________________
The presently available vehicle interlock systems have certain shortcomings which include a lack of efficacy for detecting impairments other than alcohol intoxication, including drug related impairment. Also, it is possible for an impaired driver to "trick" vehicle interlock systems by having a sober accomplis perform the breathalizer test so that the impaired driver can operate the vehicle, or to perform other physical activities which are intended to identify the suspect driver.
It would be desirable to provide a new generation of vehicle interlock systems which control access to vehicle operation, not just based on measurable blood alcohol level, but through the additional detection and quantitative measurement of non-alcoholic drug levels and even the detection of dangerous physical conditions which are detectable through analysis of blood or tissue chemistry, such as acute imbalances arising from diabetes, hypoglycemia, severe dehydration, etc. An even greater degree of societal protection would arise from such a new generation of vehicle interlock systems which positively identifies drivers and repeatedly re-tests and re-verifies identity during the course of vehicle operation.
b. Basic Concept of Interlock System.
The vehicle interlock of the present invention incorporates, in lieu of existing breathalizer user testing components of vehicle interlock systems, an optically based user testing system which non-invasively analyzes blood and tissue with respect to the presence and concentration of certain chemical constituents. A preferred embodiment of the present system will involve a device into which a monitored driver will insert a finger, ear lobe, or other bodily projection for analysis of systemic alcohol or drug concentrations.
c. Optical Analysis Aspects of System.
All wavelength components of polychromatic light are polarized, but not in the same way, and each must be examined separately. Each wavelength responds differently to a specific optically active medium.
After adding the analysis of wavelength it is advantageous to add the more complex analysis of the polarization rotational characteristics that result from the irradiation of complex solutions (such as blood). In general, organic molecules are structured in spiraled form and have a definite helicity or handedness. It is this helicity which gives a molecule its ability to rotate the polarization of the incident light. For example, dextrose (d-glucose) is, by convention, right-handed since, when viewed from the perspective of light emerging from the sample, the polarization axis has rotated in a clockwise direction. On the other hand, levulose (fruit sugar) is left-handed since it rotates the polarization axis in a counter clockwise direction. Molecules or material which exhibit this kind of optical activity are said to possess optical rotary power. In particular, these are termed dextrorotary or levorotatory respectively depending upon the action on the polarization of the incident light. The magnitude of the angle, through which the polarization direction rotates is, in simple theory, proportional to the inverse of the wavelength of the incident light squared. Sometimes called a dispersion function, this relationship has a weak dependence on wavelength but is strongly a function of the type of material or molecular structure being irradiated. This functional dependence on the physical properties of the medium manifests itself in the difference of the indices of refraction for right- and left-handed polarized light. Two circularly polarized waves of opposite helicity form a set of basic fields for the description of any general state of polarization. As a result, for example, if the polarization of the light irradiating the sample were purely elliptical not only would the ellipse rotate by about an axis parallel to the direction of propagation of the light, but the ellipse also distorts--its eccentricity changes. This latter phenomenon is called circular dichroism. It is due to the different absorption between right- and left-handed circularly polarized light.
In a fluid, where there is no long-range order, the molecules are randomly oriented. Nevertheless, the effect of rotary power is not averaged out to zero. Since the constituent molecules all have a definite helicity which is the same, they cannot be brought into coincidence with their mirror images--they are enantiomorhpous. Thus, the effect of the rotary power of an individual molecule is enhanced in a fluid state. Substances which exhibit both optical rotary power and circular dichroism are referred to as chiral media.
A glucose solution is an isotropic chiral substance. When plane-polarized light impinges normally on glucose the vibration ellipse of the transmitted light is different from the vibration ellipse of the incident light. The difference is characterized by two quantities: (i) Optical rotation (OR), which is the angle by which the transmission ellipse rotates with respect to the incidence ellipse; (ii) Circular dichroism (CD), which is a measure of the difference in the eccentricities of the two ellipses. Profiles of the OR and the CD of an isotropic chiral substance with respect to frequency are sufficiently unique that they can be used as a component in the signature of a substance to be identified and quantified. Because the OR and the CD of any substance have been shown to be Kramers-Kronig-consistent, complete knowledge of either of the two quantities as a function of the frequency is sufficient to determine the other; therefore, the more easily measured OR is often used to characterize isotropic chiral substances.
A first issue that must be addressed is that of polarization of the light incident on the biological sample whose glucose content has to be monitored. Let us suppose that the incident light is a planewave traveling in the +z direction (of a Cartesian coordinate system) at a frequency .function.. The electric field phasor associated with this planewave may be adequately set up as EQU E.sub.inc (z,t)=[A.sub.x u.sub.x +A.sub.y u.sub.y ]e.sup.-i2.pi.f(t-z/.spsp.c.sbsp.o.sup.), (1)
where t is time and c.sub.o =3.times.10.sup.8 m/s is the speed of light in free space; i=(-1); (u.sub.x, u.sub.y, u.sub.z) are the unit cartesian vectors; and A.sub.x and A.sub.y are complex amplitudes with units of V/m.
Let the complex amplitudes be independent of time t. In general, Eq. (1) then represents an elliptically polarized planewave whose vibration ellipse does not change with time t. When either A.sub.x =0 or A.sub.y =0, the planewave is said to be linearly polarized. When A.sub.x =.+-.iA.sub.y, the planewave is circularly polarized.
Suppose now that A.sub.x and A.sub.y are functions of time t. Then Eq. (1) should be rewritten as EQU E.sub.inc (z,t)=[A.sub.x (t)u.sub.x +A.sub.y (t)u.sub.y ]e.sup.-i2.pi.f(t-z/.spsp.c.sbsp.o.sup.). (2)
It still denotes a planewave, but one whose vibration ellipse changes with time t. Complicated sources have to be utilized in order to deliver specific A.sub.x (t) and A.sub.y (t) Indeed, the prior art devices utilize a complicated light source that yields A.sub.x (t) and A.sub.y (t) as controllable functions of time t.
The preferred embodiment of the present invention, however, utilizes a source based on Quartz-Tungsten-Halogen (QTH) lamp whose output in the focal region is partially polarized. Other suitable light sources include devices which emit light at multiple frequencies, such as LEDs. To understand the term "partially polarized", it is best to begin by thinking about "totally unpolarized" planewaves. The functions A.sub.x (t) and A.sub.y (t) are continuously random functions of time for a totally unpolarized planewave, therefore the rotation of a totally unpolarized planewave by a glucose cell cannot be measured and even the concept is of no meaning.
A partially polarized planewave can be thought of as a combination of a totally unpolarized planewave and an elliptically polarized planewave. The second component of the partially polarized wave suffers a definite rotation on passage through a glucose cell, therefore can be used for OR measurements.
The present invention has a source that delivers a slightly polarized planewave, thus its rotation by the glucose cell is meaningful.
A second issue that must be addressed is that of chromaticity. The devices described in the prior art ideally need monochromatic sources, i.e., sources whose outputs are fixed at precisely one frequency. Practical monochromatic sources cannot be ideal, instead their frequency range is very small.
Suppose f.sub.c is the center-frequency of a source and its 3-db bandwidth is denoted by .DELTA.f; then, we can define a quality factor EQU Q=f.sub.c /.DELTA.f. (3)
The QTH lamp used in the preferred embodiment of the present invention is a white-light lamp operating from 400 to 2000 nm with a peak at 900 nm; thus, its useful frequency spectrum ranges from 1.5.times.10.sup.14 Hz to 7.5.times.10.sup.14 Hz with its peak intensity at 3.3.times.10.sup.14 Hz. As the QTH output is roughly independent of the frequency over the operating range, we can estimate its Q=3.3/(7.5-1.5)=0.55. Thus, the QTH lamp is definitely a polychromatic source.
The present invention also utilizes a polarization-preserving analyzer whose response is flat over the 2.3.times.10.sup.14 Hz to 4.3.times.10.sup.14 Hz range, and it uses a compensated polychromatic detector to measure the intensity of the beam transmitted by the analyzer. In sum, the present invention is polychromatic (low-Q), while the devices described in the prior art are monochromatic (high-Q).
Polychromaticity has a definite advantage over monochromaticity for such things as blood glucose measurements. The OR spectrum of a chiral solute in a non-chiral solvent depends on the concentration of the solute. The amount of analyte (alcohol, drug, etc.) in the blood varies with time and from sample to sample. This means that the OR spectrum of a blood sample will shift with time as alcohol, drug, glucose, or other substances vary in concentration over time. A polychromatic system therefore has a much better chance of monitoring a continuously varying non-normoglycemic sample than a monochromatic one.
Accordingly, the user testing component of the present vehicle interlock system utilizes, not just any optical analysis methodology, but one which utilizes the unique and refined approach to optical analysis described above and distinguished from earlier approaches.